The present invention relates to a system for the control of emissions with catalytic converters, and in particular to the use of a baffle to maintain separation between converging exhaust gas streams entering a single catalytic converter to improve overall emissions performance at a low cost.
Automobile engines are designed for optimal efficiency at an ideal air/fuel ratio which produces complete combustion of the fuel. In operation, however, automobile engines fluctuate in efficiency due to variation in the air/fuel ratio from the ideal. This can produce undesirable exhaust gas components, such as carbon monoxide (CO) and unburned hydrocarbons from rich mixtures, and nitrogen oxides (NO.sub.x) from lean mixtures.
To more tightly control and reduce the production of undesirable exhaust gas components, independent control over the air/fuel ratio fed to separate groups of engine cylinders, typically the left and right cylinder banks, has been maintained using feedback from exhaust gas oxygen sensors placed in the exhaust gas streams from those groups of cylinders. The exhaust gas streams from each group of cylinders are often combined, and the combined stream passed through a catalytic converter.
The catalyst most often employed in these catalytic convertors is three-way catalyst operation, the catalyst is a combination of bonding sites for reducing and oxidizing species. The exhaust from the groups of cylinders is modulated from lean to rich in order to use the advantages of the three-way catalyst. As a rich exhaust, high in reducing species, is passed through the catalyst, the reducing species are bound in the three-way catalyst and stored. When the exhaust is modulated back toward a lean mixture, high in oxidizing species, the reducing species are destroyed and the oxidizing species bound to the catalyst. Oxidizing species are then destroyed when rich exhaust is cycled back through. The three-way catalyst thereby operates by modulating the exhaust mixture from lean to rich.
This modulation from lean to rich exhaust is regulated by the oxygen gas sensors placed in the exhaust stream. Because each group of cylinders is independently controlled, the frequency and amplitude of variation in the exhaust gas components is different for each separate exhaust gas stream. This may create problems when the exhaust gas streams from the differing groups of cylinders are combined.
Due to the varying frequency of the differing exhaust streams, sometimes the exhaust gas streams being combined are "in phase", allowing the three-way catalyst to react lean and rich exhaust gas components with peak efficiency, and sometimes they are "out of phase", subjecting the three-way catalyst to extended periods with excessive levels of either lean or rich exhaust gas components which overload its capacity for retention. Under these latter conditions, the catalytic converter can allow a surge of exhaust gas components, either CO and unburned hydrocarbons from rich mixtures, or NO.sub.x from lean mixtures, to pass through to the environment unreacted. This is commonly called catalyst breakthrough.
Japanese Patent 53-116787 to Horie et al disclose traditional engine exhaust systems. In Horie et al, the exhaust of differing cylinders is combined and passed through a catalytic convertor. The converter of Horie et al is a standard two brick catalyst. A two brick catalyst comprises two different catalysts in series, reducing catalyst in the front and oxidizing catalyst in the rear. In order to properly operate the two brick catalyst, Horie et al must run the exhaust stream rich at all times.
Horie et al employ a baffle to separate the exhaust streams coming from differing groups of cylinders. However, Horie et al's purpose is to reduce exhaust pressure to improve engine efficiency. Horie is not concerned with catalyst efficiency. Further, Horie et al do not disclose use of air/fuel controls, exhaust gas oxygen sensors, or three-way catalyst in conjunction with a baffle to separate exhaust streams in order to improve efficiency of three way catalyst systems.
Accordingly, a need exists for an exhaust system which removes the effect of the varying frequencies of exhaust gas components from differing groups of cylinders when employing air/fuel control and three way catalytic converters.